1. Field of the Invention
The present invention relates to cross-coupled filters, and more particularly, to a cross-coupled bandpass filter for generating transmission zeros in a transmission rejection band.
2. Description of Related Art
Current portable communication devices have high requirements on pass band selectivity. Quadruplet cross-coupled bandpass filters are usually used to achieve high selectivity. In general, a microstrip filter using electric cross-coupling can generate two transmission zeros in a rejection band.
FIG. 1A is a schematic view of a conventional quadruplet cross-coupled bandpass filter using electric cross-coupling. Referring to FIG. 1A, the conventional bandpass filter has four microstrip open-loop resonators 12, 14, 16, 18 formed on a dielectric substrate 11. The resonator 12 serves as an input port and the resonator 14 serves as an output port. FIG. 1B shows the frequency responses of the bandpass filter of FIG. 1A. Referring to FIG. 1B, curve C11 shows the reflection coefficient (S11) of the input port (resonator 12), and curve C12 shows that the forward transmission coefficient (S21) from the input port (resonator 12) to the output port (resonator 14) has two transmission zeros generated in the rejection band. However, during an integrated passive device (IPD) fabrication process, input and output signals are fed through capacitors. As such, capacitors connected to the input port (resonator 12) and the output port (resonator 14), respectively, are too close to each other, thereby easily causing short circuits. Since it is difficult to achieve electric cross-coupling in an IPD fabrication process so as to generate a transmission zero in a high frequency rejection band, using magnetic cross-coupling to generate two transmission zeros in a rejection band has currently become a research focus.
FIG. 2A shows a triplet magnetically cross-coupled bandpass filter 20 using an IPD fabrication process. Referring to FIG. 2A, the bandpass filter 20 has a resonator consisting of an inductor 22 and a capacitor 23a, a resonator consisting of an inductor 24 and a capacitor 25a (including a lower polar plate 25b) and a resonator consisting of an inductor 26 and a capacitor 27a (including a lower polar plate 27b). The inductor 24 and the capacitor 25a serve as a signal input port, and the inductor 26 and the capacitor 27a serve as a signal output port. Two terminals of the inductor 22 form an opening 22a, and the two terminals are electrically connected to each other through the capacitor 23a. Two terminals of the inductor 24 form an opening 24a, and the two terminals are electrically connected to each other through the capacitor 25a and a through hole 25c. Similarly, two terminals of the inductor 26 form an opening 26a and the two terminals are electrically connected to each other through the capacitor 27a and a through hole 27c. FIG. 2B shows the frequency responses of the bandpass filter 20. Curve C21 shows that the forward transmission coefficient S21 has a transmission zero generated in a low frequency rejection band, but has no transmission zero in a high frequency rejection band. Curve C22 shows that the input reflection coefficient S11 and the output reflection coefficient S22 are nearly the same in a symmetrical configuration.
As described above, it is difficult to achieve electric cross-coupling in an IPD fabrication process in the prior art. Also, it is difficult to use magnetic cross-coupling to design a bandpass filter with two transmission zeros in a rejection band. Therefore, there is a need to provide a cross-coupled bandpass filter that is applicable in an IPD fabrication process so as to effectively use magnetic cross-coupling to generate two transmission zeros in a rejection band.